This project will continue our investigation into the physiological significance of substrate cycling in the regulation of fat and glucose metabolism in human subjects. The studies of the triglyceride/fatty acid (TG/FA) cycle, and its individual components, lipolysis, and reesterification, will focus on the relationship between fatty acid mobilization and the energy requirement of the lean body mass. We will test the general hypothesis that lipolytic sensitivity in exercise and recovery is enhanced by repetitive adrenergic stimulation resulting from either (a) strenuous exercise training; or (b) daily epinephrine infusion. In contrast, we propose that moderate exercise training that does not elicit an adrenergic response will have little effect on lipolytic sensitivity. We also plan to study the interaction of adrenergic stimulation and body composition by determining the effect of moderate weight loss and weight loss to ideal body weight on lipolytic responsiveness, with and without strenuous exercise training. The second aspect of this proposal will focus on the potential cycle between pyruvate, oxaloacetate, (OAA) phosphoenolpyruvate (PEP) and pyruvate. We have developed a novel technique for assessing this cycle utilizing labeled acetate infusion, and we have also improved a more traditional approach for assessing the same factors. We will use these methods to test the following hypotheses: 1). The stimulation in net gluconeogenesis that occurs with fasting results from both a stimulation of phosphoenolpyruvate carboxykinase (PEPCK) and an inhibition of pyruvate kinase. 2). Gluconeogenesis in short term fasting is limited by fructose diphosphatase activity. Increased supply of substrate that enters the gluconeogenic pathway below this step will competitively inhibit net gluconeogenesis from other substrates, in part by direct enzyme inhibition (i.e., glycerol kinase or PEPCK), and in part by stimulation of pyruvate kinase.